The present invention relates to a matrix display system comprising a plurality of row and column conductors, a plurality of picture elements each comprising a liquid crystal display element connected in series with an associated two terminal non-linear resistance element exhibiting a threshold characteristic between a row conductor and a column conductor, and drive signal generating means for applying drive signals to the row and column conductors for driving the display elements, the drive signal supplied to one of the two conductors associated with each picture element consisting of a selection signal portion during which the display element is set to a desired display condition and a sustain signal portion for sustaining that display condition during a subsequent interval prior to the picture element receiving a further selection signal portion.
An active matrix display system of this kind is suitable for displaying alpha-numeric or video, e.g. TV, information.
Display systems of this kind in which the non-linear resistance elements comprise diode structures are known.
In FIGS. 1(a) and 1(b) of the accompanying drawings, there are shown diagrammatically two examples of the basic circuit configuration of a typical picture element and its associated row and column conductors of a known form of such a liquid crystal display system. In these circuits, each liquid crystal display element 12, constituted by a pair of spaced electrodes with liquid crystal material therebetween, is connected in series with a diode ring type of non-linear resistance element 14, comprising in these examples a pair of diodes connected in parallel with opposing polarities, between a row, scanning, conductor 16 and a column, data, conductor 18. The two forms of circuit configurations shown are electrically equivalent and perform in the same manner. The choice between them is made purely on technological grounds.
The transmission (T)-RMS voltage (Vlc) curve of the liquid crystal material, the current (I) voltage (V.sub.R) characteristic of the diode ring and the drive waveforms applied to the row and column conductors are illustrated in FIGS. 2, 3 and 4(a) and 4(b), respectively.
The purpose of the diode ring is to act as a switch in series with the display element. When a given row of the display is to be driven, the voltage applied to the row conductor concerned, illustrated by the waveform of FIG. 4a, is taken to one of two selected levels Vs. In common with most other liquid crystal display systems the polarity of the voltage applied across the liquid crystal display element is inverted every field. Since the operation of the picture elements in the positive and negative cycles are exactly equivalent, the following discussion will consider a cycle of only one polarity for simplicity.
During the "select" period t.sub.g (FIG. 4a), corresponding in the case of TV display to a maximum of a line period, the voltage across the diode ring and display element causes the diode ring to operate in the charging part of the diode ring characteristic, indicated at C in FIG. 3. In this region the diode ring current is large and the display element capacitance rapidly charges to a voltage, Vp, given by the expression: EQU Vp=Vcol-Vs-Vd, (1)
where Vcol and Vs are respectively the voltage applied to the column conductor 18 at that time and the select voltage applied to the row conductor 16, and Vd is the voltage drop across the diode ring. Vcol is derived, in the case of a TV display, by sampling the appropriate line of the incoming video signal, in accordance with known practice. At the end of the select period t.sub.s the row voltage falls to a new, lower, and constant value Vh (FIG. 4a) which is selected so that the mean voltage across the diode ring during the next approximately 20 milliseconds, corresponding to the usual field period for TV display less the duration of the period t.sub.s, when the row is next addressed again with a select voltage, is minimised. In theory, assuming an ideal situation, this sustain, or hold, voltage Vh is equal to the mean of the rms saturation and threshold voltages (as shown in FIG. 2), that is: EQU Vh=(Vsat+Vth)/2. (2)
Under these conditions the maximum voltage of either polarity appearing across the diode ring is equal to the peak-to peak voltage on the column conductor, which in turn is equal to the difference between the rms saturation and threshold voltages Vsat and Vth. As the voltage across the diode ring increases, larger leakage currents flow through the diodes and vertical crosstalk appears. For a given level of display performance it is possible to derive a maximum acceptable diode voltage which is shown at Vdm in FIG. 3. This means that the display will only operate correctly if the condition: EQU Vsat-Vth&lt;Vdm (3)
is satisfied. Vdm can be controlled by placing several diode rings in series or by varying the way in which the diodes are fabricated so that the slope of the diode I-V curve is changed. The latter approach only allows small changes to be produced so the main way in which the diode ring characteristics can be matched to the liquid crystal is to place a number of diode rings in series until Vdm for the combination satisfies the above equation. Two examples of the circuit of a typical picture element employing a number of diode rings in series as the non-linear resistance element is shown in FIGS. 6(a) and 6(b).
Clearly, the smaller the difference between Vsat and Vth, the fewer diode rings are needed. However, a certain minimum difference is needed to allow grey scale levels to be accurately reproduced. The use of a minimum number of diode rings is desirable for two reasons. Firstly, the chances of producing a faulty diode increase as the number of diodes increases and so the yield of good displays becomes lower as numbers increase. Secondly, for a display device operated in the transmission mode, and bearing in mind that the diodes are usually fabricated side by side and situated adjacent an electrode of their associated display element on a substrate of the device, the effective optical transmission area of the display becomes smaller as more diodes are used, making the display dimmer for a given backlight power.
It has been found that in operation the known display system can exhibit unwanted vertical cross-talk effects and that the minimum number of series connected diode rings necessary for acceptable performance in reducing the level of cross-talk exhibited is greater than the number expected as a result of the above theoretical considerations. Because of this, the display system is likely to suffer more than expected from the above described problems.